Constrain release-notes doc links to existing files

[frontegg-david]

The v2.6.1 release notes linked to docs/overlay.md, which does not exist (overlay docs live in docs/curation.md) — the AI notes generator constructed a plausible filename instead of a real one. The published release body has already been corrected in place.

This PR prevents the next occurrence: the notes-generation step now reads the actual docs/ directory listing at generation time, injects it into the prompt as the only valid link targets, and the link rule requires filenames copied verbatim from that list instead of "files that appear in the diff" (which the model cannot reliably verify).

Test plan

• Workflow YAML validated after the edit
• Prompt-only change to publish-release.yml; no library code touched

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Cherry-pick Created

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